I'm not sure you understand how simple it is...
Check out a few sites.
www.invisionize.com offers TONS of hacks, and they give you step by step instructions on how to install them. All you have to do is back up the .php file you are editing, and if something goes wrong, just re-upload the original. This site also gives a ton of skinning advice, etc.
Another good site is
www.invisionthings.com. They also offer a ton of support, and free downloads. They have also made very handy skinning guides, that teach you how the system works.
Seriously...if you're afraid of going under the hood and messing something up, it shouldn't be a concern. All you have to do is backup a file before you start editing it, and all is well.
If you end up paying someone to do it for you, I'd just go with Inivision itself. Pay for the subscription, and they will do everything for you.